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Abstract. Molecular singlet oxygen (1O2*) is the first excited state of molecular oxygen (O2) and can be formed through indirect photochemistry during irradiation of chromophoric organic matter. Once formed in the particle and droplet phases in the atmosphere, 1O2* can be a competitive oxidant in the photochemical processing of organic matter. Now, as more researchers study the atmospheric photochemistry of 1O2*, establishing protocols by evaluating and comparing experimental setups across laboratories is becoming necessary. Here, we present 1O2* measurements from four photosensitizing molecules in four photoreactor setups at three research institutions, including two xenon lamps of different strengths and two multi-bulb UVA + UVB broadband systems. The production of 1O2* was investigated from perinaphthenone, lignin, and juglone, which are photosensitizers with atmospherically relevant light absorbing moieties, as well as from Rose Bengal, a standard photosensitizer. Two chemical actinometers, 2-nitrobenzaldehyde and p-nitroanisole/pyridine, were used to quantify photon fluxes and calculate rates of light absorbance for photosensitizers for each photoreactor. We compared two commonly used 1O2* quantification methods, chemical probe method using furfuryl alcohol, as well as direct 1O2* phosphorescence detection at 1270 nm. Rates of light absorbance across experimental setups for each photosensitizer ranged between 0.2 and 62 × 10−5 molphotons L−1 s−1, while 1O2* steady-state concentrations ranged between 0.01 and 129 × 10−11 M. Despite order of magnitude differences in rate of light absorbance and 1O2* steady state concentrations, normalizing to 1O2* quantum yields showed good inter-laboratory agreement but only for the sensitizers with high quantum yields: perinaphthenone (94 % ± 9 %–112 % ± 17 %) and for Rose Bengal (67 % ± 15 %–87 % ± 5 %). 1O2* quantum yields for lignin and juglone increased with decreasing irradiation wavelength, highlighting a wavelength-dependence. Finally, we make five recommendations to improve the accuracy and reproducibility of 1O2* measurements for the atmospheric chemistry community. These recommendations include considering wavelength-dependent quantum yields, avoiding suppression of 1O2*, controlling and reporting photoreactor temperature, considering light scattering from nanoparticles, and conducting control experiments. These recommendations can serve as guidelines to build future photoreactors as well as help standardize 1O2* measurements in studying photochemical processing of atmospheric aerosols and droplets.
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Photooxidation and Cleavage of Ethynylated 9,10-Dimethoxyanthracenes with Acid-Labile Ether Bonds
This paper describes a series of twelve 9,10-dimethoxyanthracene derivatives functionalized with a range of electronically diverse ethynyl substituents at the 2 and 6 positions, aimed at tuning their optoelectronic properties and reactivity with singlet oxygen (1O2). Optical spectroscopy, cyclic voltammetry, and density functional theory calculations reveal that the ethynyl groups decrease the HOMO-LUMO gaps of these acenes. Notably, bis(dimethylanilineethynyl) substituents increase the wavelength of absorbance onset by over 60 nm compared to 9,10-dimethoxyanthracene (DMA). Furthermore, all twelve molecules react with 1O2 through cycloaddition at the 9 and 10 positions to form endoperoxides. Although the presence of ethynyl groups decreases the reaction rates, they are at least 40% of the rate observed for DMA. Finally, these endoperoxides cleave to form quinones when exposed to protic acid. This behavior, combined with red-shifting of absorbance spectra, emphasizes their potential in photocleavable materials.
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- Award ID(s):
- 2003341
- PAR ID:
- 10621699
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- The Journal of Organic Chemistry
- Volume:
- 90
- Issue:
- 5
- ISSN:
- 0022-3263
- Page Range / eLocation ID:
- 1996-2007
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/acs.joc.4c02737
